1. Field of the Invention
Many steam turbines suffer damage from the passage therethrough of particulate material. The particulate material may come from the boiler, such as spalled oxide from pipes, or as weld beads or slag that is produced when boiler tubes are repaired.
Broken turbine parts are another source of such particles.
These materials roughen and damage the nozzle and bucket profiles and degrade turbine efficiency.
The damage is increased when the particulate material is permitted to become entrapped above the bucket covers and ahead of stationary tip seals. This is a location where the particulate, due to circular motion, is centrifuged to the outer periphery where it cannot easily escape It is normally forced to circulate many times before a lucky bounce allows it to escape. While circulating, the particles bounce between the stationary parts (tip seals, side walls, etc.), and the rotating parts (bucket covers and tenons), causing damage that increases leakage area and threatens the ability of the tenons to hold the bucket covers. The result is poorer efficiency and reliability.
2. Description of the Prior Art
Manufacturers try to prevent the formation and entry of such particulate and use hardened surfaces on nozzles and buckets to minimize damage; but severe damage still occurs in the vicinity of the tip seals and bucket covers.